Why can a child have a genetic disorder with no family history of that disorder?

A child may be born with a genetic disorder despite having no family history of the condition for several reasons:

• New Mutations: A disorder can arise from a de novo mutation that occurs for the first time in the egg or sperm cell, meaning it was not inherited from either parent.1
• Autosomal Recessive Inheritance: In these cases, both parents are typically asymptomatic carriers, each possessing one mutated allele and one normal gene. Because the parents do not show symptoms, the disorder appears to emerge unexpectedly in the child.2
• Small Family Size: Sometimes a disorder is hereditary, but it appears sporadic simply because the family size is too small for the condition to have manifested in previous generations.1
• Genomic Imprinting and Uniparental Disomy: Some disorders involve genomic imprinting, where gene expression depends on the parent of origin. In rare instances, a child may inherit both copies of a chromosome from one parent (uniparental disomy), which can lead to the loss of necessary gene expression or the inheritance of two mutant copies from a single carrier parent.13
• Chromosome Abnormalities: Many chromosome abnormalities occur sporadically and are not inherited from the parents.1